How to select Distinct values from Datatable & update the records

Private Sub ProcessReturnIdentifierInResponse(ByRef dtAvailResult As DataTable)
            Try
Dim filterCondition As StringBuilder = New StringBuilder(String.Empty)
Dim returnIdentifierCounter As Integer = 1
Dim rowsWithUniqueReturnIdentifierValue() As DataRow = dtAvailResult.DefaultView.ToTable(True,AirlineAvailibilityConstants.Avail_ReturnIdentifier).Select()
For Each uniqueReturnIdentifierRecord As DataRow In rowsWithUniqueReturnIdentifierValue
filterCondition.Clear()
filterCondition.Append(AirlineAvailibilityConstants.Avail_ReturnIdentifier)
filterCondition.Append("=")
filterCondition.Append(uniqueReturnIdentifierRecord(AirlineAvailibilityConstants.Avail_ReturnIdentifier).ToString())
Dim filteredRows() As DataRow = DTAvailResponse.Select(filterCondition.ToString())
For Each recordToUpdate As DataRow In filteredRows
recordToUpdate(AirlineAvailibilityConstants.Avail_ReturnIdentifier) = returnIdentifierCounter
returnIdentifierCounter += 1
Next

Next

            Catch ex As Exception

            End Try
        End Sub